1. Technical Field
The present invention relates to a vessel mooring system with active control and more specifically to a system for monitoring mooring loads applied to and displacement of a vessel. In particular although not solely the invention relates to the control of a mooring system employing mooring robots having an attractive attachment element for engagement with a surface for making fast the ship.
2. Background Art
The mooring of a ship at a terminal such as a dock utilising mooring robots is known. Automated systems such as these are described for example in WO 0162585 and have a number of advantages over conventional methods of mooring employing mooring lines.
When a ship is approaching the terminal mooring robots are able to secure a ship and subject it to large forces within a reasonably short time to counter any significant dynamic forces in order to reduce movement of the ship and thereby bring it under precise control into a desired position relative to the terminal. However, a problem which any mooring system must counter is the effect of water currents and wind which tend to apply forces to a ship in a direction which may encourage the ship out of contact with the mooring robots. This introduces important safety consideration in the design of robotic systems employing attractive attachment elements such as vacuum cups. In considering environmental aspects, it is desirable to provide a high level of safety while also avoiding over-design and excessive redundancy.
Failure in the mooring of a vessel with a vacuum cup style mooring robot occurs when the forces applied to a vessel in a direction tending to release the vessel from the vacuum cups exceed the suction force of the vacuum cups on the vessel. This holding force can vary according to the degree of suction that is applied by the pneumatic suction system. The size of the holding force and hence the holding capacity applied by the mooring robots to the vessel can hence vary. In more traditional forms of mooring using mooring lines, the holding capacity provided by the mooring lines is determined by the break strength of the mooring lines or the strength of the fixtures holding the mooring lines between the vessel and the shore.
In conventional mooring methods employing mooring lines, various methods have been proposed for monitoring the mooring loads and controlling the mooring system to avoid catastrophic failure. For example, the magnitude of the tensile loads in the mooring lines have in previous methods been monitored to control automatic mooring winches. For example U.S. Pat. No. 4,055,137 describes the use of tension detectors to determine the tension force within a mooring line connected between a wharf and a vessel. Such information is used to control the winches to make adjustments to the tension of the mooring lines as desired. The system of U.S. Pat. No. 4,055,137 may however only be relied upon to ensure that certain limits of force within the mooring lines are not exceeded. Such limits are fixed dependent on the tensile strength of the mooring lines or fixtures in question. Since such tensile strength limits do not vary over time and cannot vary over time the information gained from the forces within the mooring lines relate only to the determination of the ultimate maximum breaking strength of the mooring. Furthermore since there is no measurement of angles of force between the vessel and the mooring lines it is not possible to utilise the system of U.S. Pat. No. 4,055,137 to determine the total force being applied to the vessel in for example the athwartship direction and longitudinal direction. Furthermore in light of there being no angular or displacement measurements being provided by the system described in U.S. Pat. No. 4,055,137 the invention of U.S. Pat. No. 4,055,137 does not allow for accurate position information to be provided as part of the system. The system of U.S. Pat. No. 4,055,137 is also unable to provide mooring load data while the vessel is moving relative to the terminal, since the system is not designed to purposefully move a ship.
U.S. Pat. No. 4,532,879 describes a mooring robot which is directly coupled to a vessel. Like U.S. Pat. No. 4,055,137, no vacuum connection is provided. Whilst a mooring force is measured in one direction only by the mooring robot of U.S. Pat. No. 4,532,879 the purposes for such is to restore the positioning of the vessel relative to the mooring robot. The force is measured to control a hydraulic pressure system to provide such restorative force. Since the ultimate holding capacity of the mooring robot is determined from the strength of the physical structure there is no need for a control of the mooring force dependent on any variation in ultimate holding strength of the coupling between the ship and the mooring robot since there is no such variation. Furthermore the mooring robot of U.S. Pat. No. 4,532,879 is capable only of measuring forces in one direction since the robot is free rotating about a pivot point. Since the mooring robot provides no lateral constraints to the ship this system is analogous to the measurement of force in a mooring line as for example shown in U.S. Pat. No. 4,055,137.
Our own prior publication of WO02/090173 describes a mooring robot however no reference is made to a relationship existing between the variable vacuum cup holding force and the forces measured by the mooring robot in at least the athwartship and longitudinal directions.
A further issue in respect of the monitoring of forces and displacements in a mooring line mooring system is the fact that such mooring lines are often elastic in nature. Accordingly no absolute determination of forces and positioning can be measured in such an elastic coupling. Whilst measurements of the mooring lines can be achieved to provide absolute information thereof, it is not an instantaneous reflection of the loading and position of the vessel.
Accordingly some of the prior art systems as described above utilise the force measurement provisions for ensuring the maintenance of the mooring system to within limits of self destruction. This is so because of the direct mechanical coupling of the vessel with the mooring robots to the wharf.
Moreover the accuracy achievable with the mooring line prior art systems is limited by the properties of the mooring lines, which may interfere with one another or with bollards etc to produce anomalous effects which cannot be readily measured.
It is accordingly an object of the present invention to provide a mooring system with active control which address the foregoing needs and problems or at least to provide the public with a useful choice.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.